1. Field of the Invention
This invention relates to a telephone system, and more particularly to a multifunction telephone system having functions such as a facsimile transmission and automatic recording.
2. Description of the Related Arts
In recent years, with the progress in the public telephone network, the development of telephone systems having the functions of facsimile transmission and automatic recording have been popular. In this type of telephone system, it is necessary to incorporate a central processing unit (hereinafter referred to as a CPU) having the capacity of processing signals in order to realize the apparatus compact in size and having a lot of functions.
FIG. 2 of the accompanying drawings is a schematic diagram showing the circuit configuration of a conventional telephone system. A KEY signal output from a key board 3 of a telephone system 1 is transferred to a CPU 4 which controls the operation of the entire telephone system. The CPU 4 outputs a CPU signal, which is a control signal, to an input controller 21 in a selected signal transmitter circuit 2 in response to the input of the KEY signal.
The selected signal transmitter circuit 2 consists of a single-chip large scale integrated circuit (hereinafter referred to as an LSI), and the input controller 21 outputs dial data to both a dual-tone multifrequency (hereinafter referred to as a DTMF) signal generator 24 and an output controller 25 in response to the CPU signal. The DTMF signal generator 24 outputs a DTMF signal composed of a combination of two frequencies on the basis of the input dial data along a telephone line. At the same time, the output controller 25 outputs a dial pulse (hereinafter referred to as a DP) signal along the telephone line on the basis of the input dial data. The selected signal transmitter circuit 2 further includes a memory 23 which stores the dial data output from the input controller 21 and outputs the dial data that has been stored and is read out again at the time of redialing to both the DTMF generator 24 and the output controller 25.
The controller CPU 4 is driven by an external power supply of +5 V, while each component in the selected signal transmitter circuit 2 is driven by a telephone line current acquired from the telephone line via a resistor 29 and a diode 28.
Thus, in the conventional telephone system 1, the CPU 4 processes the KEY signal entered from the key board 3 and outputs a control signal to the selected signal transmitter circuit 2, and then transmits either the DTMF signal or the DP signal along the telephone line. In the case where the external power supply drops due to, for example, a power failure, the CPU 4 will not operate on the telephone line current due to the large power dissipation of the CPU.
As alternatives, it may be considered that the KEY signal is directly delivered from the key board 3 to the input controller 21 in the selected signal transmitter circuit 2. In such a circuit configuration, however, it is difficult to effect a compaction function by which a given telephone number is defined by a predetermined compacted telephone number and output to the input controller 21 because the KEY signal is not processed by the CPU 4.